The present invention relates generally to the field of nanotechnology. More specifically, the invention provides a method and system for making nanoparticles using an atmospheric-pressure plasma microreactor. Merely by way of example, the invention has been applied to making silicon nanoparticles, but it would be recognized that the invention has a much broader range of applicability.
The promise of silicon-based optoelectronics has spurred intense interest in silicon nanoparticles (np-Si). Direct band gap transitions have been observed for np-Si. For example, stable photoluminescence (PL), tunable in the range between 700 and 350 nm, has been reported for np-Si smaller in size than the excitonic radius for bulk Si, which is about 4 nm. np-Si have been produced using a variety of techniques including colloidal growth, aerosol processes, plasma synthesis, and electrochemical etching. Many of these techniques involve a capping agent for protection from uncontrolled oxidation which, however, may introduce surface recombination states that alter the emission characteristics. From this perspective, aerosol techniques have an advantage since particles can be grown without capping agents and deposited directly onto a substrate.
FIG. 1 shows simplified conventional aerosol processes. Within an aerosol flow reactor, the following processes occur at different time scales and locations. For example, initial nucleation of particles results from the formation of a supersaturated vapor of gas precursors. Possible means of generating a vapor source include pyrolysis, laser ablation, spark ablation, and plasmas. In the early stages, particles grow by condensation of vapor at their surface and coalescent coagulation. Normally, these processes occur in a region near the vapor source where the temperature is high. As the particle concentration increases, collisions between particles become more frequent and agglomeration begins. Formation of these undesirable aggregates is usually found away from the vapor source as the temperature drops off. Hence the particles synthesized by the conventional aerosol processes often have a broad size distribution, which often necessitates post-synthesis size-selection and particle agglomeration. Notably, production of blue-light emitting np-Si has been challenging because of difficulties in limiting aerosol growth to small sizes and preventing particle coagulation.
Hence it is desirable to improve techniques for making silicon nanoparticles.